The present invention relates to a system for mapping the locations of wafers within a cassette which has been loaded into a wafer processing tool. More particularly, the present invention relates to a vision system utilizing front lighting for mapping the locations of wafers and detecting mispositioned wafers in a wet environment.
The manufacture of semiconductor wafers, magnetic disks, and other workpieces typically involves many processing steps. Wafers are sliced from a silicon ingot, polished, and cleaned. Often microelectronic structures are applied to the wafer, followed by application of a layer of dielectric materials. Between processing steps, the wafers are commonly transported between processing stations in cassettes containing a plurality of slots, usually 25, for holding the wafers. Often, the cassettes are carried from a wet output tool, such as a Chemical Mechanical Polishing (CMP) tool, and thus arrive at the next processing station in a wet condition. This is known as a xe2x80x9cwet environmentxe2x80x9d, a condition which is exemplified when wafers are transferred between xe2x80x9cAurigaxe2x80x9d polishers and xe2x80x9cCapellaxe2x80x9d cleaning systems, both of which are manufactured by SpeedFam Corporation of Chandler, Ariz.
As the wafers are loaded into a cassette in the wet output tool, positioning errors may occur. Wafers may end up in a xe2x80x9ccross-slottedxe2x80x9d position, in which one side of the wafer rests in a first slot and the opposite side rests in a second slot; or they might also assume a xe2x80x9cdouble-slottedxe2x80x9d position, in which two wafers occupy one slot within the cassette.
Severe problems can arise if cross-slotted or double-slotted wafers are not detected and corrected before wafer processing commences. Cross-slotted wafers may be difficult or impossible for the transport mechanism to move. With respect to double-slotted wafers, the robot arm may retrieve both wafers and then attempt to simultaneously forward the wafers to the next processing station or step. Simultaneous processing of two wafers may result in the tool becoming jammed or wafers being destroyed. In view of these concerns, systems have been developed for detecting cross-slotted and double-slotted wafers within cassettes.
Numerous approaches have been developed for detecting cross-slotted and double-slotted wafers. In one approach, a laser beam is focused upon each slot in a cassette. If a wafer is present, the beam is ideally reflected off the edge surface of the wafer into the laser""s receptacle. To determine whether a wafer is cross-slotted, the laser approach often requires two laser beam scans. The first scan may be focused upon the left edge of the slot while the second scan may be focused upon the right edge, or vice versa. Double scanning may reveal cross-slotted wafers as the reflected laser beam during the first scan will often appear at a different height than the height of the reflected laser beam during the second scan, relative to the plane of the cassette slot being scanned. For a properly slotted wafer, the height of each scan should be approximately the same.
When wafers are double-slotted, however, the laser may not detect the presence of both wafers. Since this approach relies upon the reflection of a laser beam back into the laser, double-slotted wafers may change the incident angle such that the laser beam may shine upon the wafer at an angle other than 90 degrees. As such the laser beam may be reflected away from the laser""s receptor, and the presence of double-slotted wafers may not be detected. Complex algorithms are often used to determine the thickness of the object being detected. In a wet environment a water droplet may reflect enough light to cause a false reading.
Use of lasers is also problematic due to the absorption characteristics of the wafers. Upon shining a white light source upon the edge of a silicon wafer, one will commonly observe a rainbow of colors. As the wafer is rotated these colors may change. This phenomena is due to the light absorptive characteristics of materials in general, and silicon wafers in particular. Each color seen on the edge of the wafer indicates that a particular wavelength of the electromagnetic spectrum is being absorbed by the wafer. For white light sources, these absorptions do not usually pose a problem; the entire spectrum is not absorbed, and some of the incident beam is reflected back towards the laser""s receptor. For fixed wavelength laser sources, by contrast, absorption can create significant problems. If the absorbed wavelength constitutes a significant portion of the incident beam generated by the laser, the magnitude of the laser beam reflected off the wafer surface may be reduced to such a level that the laser""s receptor does not receive enough light to detect the wafer""s presence. Lastly, while multi-wavelength lasers are commercially available, their prohibitive costs often make them impractical to utilize for wafer detection.
The laser approach in a wet environment is even more problematic. Due to the refractive nature of wet solutions, the laser beam may be slightly refracted by the solutions. These refractions may prevent the initial laser beam from hitting the wafer at the designated incidence, prevent the reflected laser beams from reaching the laser""s receptors, or reflect more light than is normally reflected by a single wafer and thereby appear to the laser receptor as a double slotted wafer. Additionally, wet wafers have a tendency to adhere to the roof of each slot due to surface tension between the wafer and the wet solution. To compensate for this tendency, the entire slot must be scanned by the laser to ensure that adhered wafers are not missed. This drastically increases the amount of time which is spent mapping wafer locations. This time increase is multiplied since each of the 25 slots must be scanned at least once, and often twice, to determine in which slots wafers are located and their orientation. Thus, the laser approach has many disadvantages: it is slow, susceptible to absorption and refraction, and works poorly in wet conditions.
Another approach utilized is an optical vision system utilizing xe2x80x9cback lightingxe2x80x9d, such as the IVS mapping system manufactured by Acuity of Cincinnati, Ohio. In back lighting systems, a light positioned behind the cassette is shined through the cassette to generate a silhouette of each wafer. The silhouettes are captured by a camera, and digitally processed. Silhouettes of cross-slotted wafers typically appear as diagonal lines, while silhouettes of double-slotted wafers are usually thicker than those of single-slotted wafers. Such vision systems typically include processing routines that analyze the silhouettes on a pixel-by-pixel basis and alert the operator when a non-nominal condition is detected. While these systems have generally been reliable fort mapping and tracking wafers in dry environments (that is, processing environments in which the wafers enter the tool from the cassette in a dry state), they have not been reliable in wet environments.
In wet environments, liquid droplets often adhere to the surface of the wafer. When a light is shown through the cassette to create a silhouette of each wafer, the droplets diffract the light and thereby appear to the vision system as a thicker-than-normal silhouette. A false detection of a double-slotted condition may result, requiring the operator to shut processing down, remove and visually inspect the cassette to confirm that each wafer is correctly positioned within the cassette, reload the cassette, and then restart wafer processing. During this operation, the operator must also ensure that no solution is allowed to dry upon the surfaces of any of the wafers, as surface irregularities and contaminations may occur when wet solutions dry upon silicon surfaces and require the wafer to be scrapped.
Another disadvantage attendant to back lit vision systems is the tremendous heat generated by the light source. In order to distinguish between the wafer silhouettes and the illuminated gaps between the wafers, sufficient light must be shown through the cassette to create the distinguishable silhouettes. 150-watt bulbs are commonly utilized. The heat generated from these bulbs increases the likelihood of the solutions drying upon the wafers prior to the wafers"" entrance into the cleaning solution. Back lit systems also pose significant electrical insulation obstacles. Due to the wet environment, and the significant voltages required by the vision systems, substantial efforts must be taken to ensure that the system (including the lamps, and cameras) is not electrically shorted.
A wafer mapping system is thus needed that provides for efficient wafer mapping in both wet and dry environments.
The present invention provides a wafer mapping system which overcomes the noted shortcomings of the prior art and provides a system and a method of automatically determining the location and orientation of workpieces within a workpiece processing tool.
One advantage of the present invention is the provision of a system for accurately mapping wafers in a wet environment. An operator can precisely determine which wafers are cross-slotted or double-slotted in a wet wafer cassette without having to remove the cassette from the tool. A white light is focused on the front edge of the wafers and is reflected off the edges into a vision system positioned to receive the reflected light. Hence, instead of generating shadows for analysis, the system generates light reflections. Moreover, since white light occupies a full spectrum, the impact of absorption of some of the spectrum by the wafer edges is minimized and a sufficient quantity of the incident light is reflected into the vision system. The absorption concerns associated with prior art laser imaging systems are eliminated.
Another advantage of the present invention is that it permits identification of a plurality of wafer positions and orientations simultaneously. Unlike prior art laser scanning systems, an optical vision system can be configured to monitor a prescribed and constant field of view. A white light is shined on the field of view and a xe2x80x9csnap shotxe2x80x9d of the field of view is captured by the vision system. The snap-shot is digitally processed and analyzed to determine the locations and orientation of each wafer within the field of view. The time-consuming need to scan an entire wafer slot is eliminated as an entire field of cassette slots is captured with one snap-shot. Preferably, the field of view encompasses five cassette slots per snap-shot.
The present invention also eliminates the need for multiple scans per wafer slot required by prior art laser scanning systems. Complete images of multiple wafer positions are captured in one shot instead of the two or more scans per individual wafer position required by laser scanning systems. Consequently, less time is spent mapping and higher rates of wafer throughput are obtained. Decreased mapping time also reduces the likelihood of solutions drying on wet wafer surfaces prior to their insertion into the cleaning solution, thereby reducing the wafer scrapping rate as well.
A further advantage the present invention is removal of the imaging system from the wet environment. Laser systems often must be positioned in close proximity to the wafer cassettes in order to eliminate the distorting effects of the solutions within the wet environment. Similarly, back-lit vision systems often must be positioned relatively close to the cassette to improve detection of silhouettes and to prevent reflections off other surfaces from xe2x80x9cfilling-inxe2x80x9d the silhouettes. The present invention, conversely, relies on reflection of white light, and does not need to be positioned directly in the wet environment in order to obtain accurate measurements. A transparent medium can be used to physically separate the vision system from the wet environment and thereby further reduce the probability of electrical shorts and equipment failure due to the wet environment.
Yet another advantage of the present invention is reduction of the heat generated by the system through utilization of fiber optic light sources to focus the light upon the wafer edges. Fiber optic light sources physically separate the light generating device and hence, the heat generated by the device, from the emission point of the light towards the wafers. The present system also allows a plurality of fiber optic beams to be generated by as few as one source, resulting in savings in equipment costs and eliminating redundancies in lighting systems.
The fiber optic light source may be enhanced or replaced by a strobe light source. Images of wafer locations and orientations are accurately obtained by timing the capture of the image with initiation of the strobe light. Initiation of the strobe light floods the field of view with light, thereby increasing the incident light reflected off the wafer surfaces such that the amount of reflected light is significantly greater than the ambient light entering the lens of the vision system. These and other aspects of the present invention are described in fall detail in the following description, claims and appended drawing figures.